1. Field of the Invention
The present invention relates generally to an apparatus for sensing position and more particularly to an optical system for resolving the position and approach angle of a moving assembly with respect to a fixed optical assembly.
2. Description of the Prior Art
The automatic transportation of particular objects from point to point within an enclosed environment is desirable in a number of industrial settings. Most notable of these includes transportation of silicon wafers from machine to machine in a clean room of a chip fabrication plant. Subsequent processing, for example, encapsulation of the chips may also be performed in a clean room and/or under semi-automatic conditions. Transportation of biohazardous or radioactive material under commerical, industrial or research conditions is also common. The environments in which these materials are found and utilized typically constitute single large rooms, e.g. clean rooms or isolation rooms, which are maintained under controlled conditions. Human manipulation is often contraindicated for reasons of safety, cleanliness, positive handling and labor costs.
These environments may have a number of machines at fixed locations within the room, to which and from which objects must be transported. Using the chip fabrication process example, it is often necessary to transport cassettes filled with wafers to and from several different processing machines. Depending on the types of chips being manufactured, the number and order of machines used will vary. Thus a fixed transportation system, e.g. a continuous belt system, may be impractical. Fixed systems additionally may unduly crowd the workplace, creating difficulties with access to the equipment.
Accordingly, a preferred mode of wafer transport within a fabrication clean room is a mobile transport unit, able to independently transfer cassettes filled with wafers between machines as needed. Such a unit would be self-propelled and computer directed to enable it to service any machine within the fab area. A robot arm would provide the means by which wafer cassettes are transferred to and from the machine and transport unit. Because the manipulations needed to transfer such cassettes require a high degree of positional control, it is advantageous to provide the mobile transport units with a positional sensing device which is independent of the main computer and which can direct the "docking" of the mobile unit with a machine.